It is known to use workholding devices to hold a workpiece still whilst performing manufacturing processes, such as machining, or whilst inspecting the workpiece.
The most common methods of workholding, i.e. mechanical clamping, vacuumed chucks and magnetic systems, all possess some limitations, particularly when delicate or difficult to hold parts require processing which involves mechanical loading (e.g. machining).
Several non-conventional workholding technologies have been developed to address some of the limitations of the common methods described above.
For example, low melt alloys, adhesives, resins, waxes, and water-ice have been employed to encapsulate irregular or delicate parts in order to ease manufacture. However, encapsulation is often undesirable as the parts have to be subsequently removed from the encapsulation media. Furthermore, whilst low melt alloys such as lead-bismuth-tin composition are stronger than waxes, small amounts of the alloy may be left on the workpiece (i.e. a turbine blade), causing corrosion problems.
It is also known to use magneto rheological fluids to hold parts during machining operations. With such fluids, the parts are locked in place when a magnetic field is applied to the fluid and are released after removal of the magnetic field. However, such systems are complex and thus expensive. The components are also susceptible to gradual movement if a continuous force is applied.
Adhesive based fixture techniques may also be used for holding a workpiece. For example, Photo-activated Adhesive Workholding (PAW) employs an adhesive which is activated by light. Such adhesives have a short cure time and provide a strong bond. However, it is necessary to de-bond the workpiece from the workholding device after the machining operation has been completed.
An example of a PAW technique is the Light Activated Adhesive Gripping (LAAG) system. The LAAG system uses an ultraviolet spot Lamp and light guide to direct UV light through gripping pins (fastening elements or pins) to rapidly cure a photo-activated adhesive applied to the gripping pin surface. The workpiece, which is loaded against the adhesive on the gripping pins, is held in place during machining by the adhesive bond. The workpiece may be de-bonded from the gripping pins by mechanically breaking the bond or using laser radiation to break the bond. Residual adhesive may be removed from the workpiece and gripping pins by mechanical abrasion.
FIGS. 1 and 2 show gripping pins used in the LAAG system which allow the workpiece to be de-bonded from the gripping pin by mechanically breaking the adhesive bond.
Specifically, FIG. 1 shows a workholding device comprising a base plate 1 and a rod or gripping pin 3. The gripping pin 3 projects above the base plate 1 and supports a workpiece 5. A photo-activated adhesive 7 is disposed between the gripping pin 3 and the workpiece 5. The gripping pin 3 has an optically transparent inner portion 9 which transmits light to the adhesive 7 so as to cure the adhesive 7, thus forming a bond between the gripping pin 3 and the workpiece 5.
After the required manufacturing processes have been completed, it is necessary to remove the workpiece 5 from the gripping pin 3. The base plate 1 and gripping pin 3 have complementary screw threads 11, and a head 13 of the gripping pin 3 is engaged by a motor 15 which causes the gripping pin 3 to rotate and translate away from the workpiece 5. This action mechanically breaks the adhesive bond between the gripping pin 3 and the workpiece 5.
However, the arrangement of FIG. 1 requires that the workpiece 5 is sufficiently supported so as to prevent rotation and translation of the workpiece 5 with the gripping pin 3. This may not always be possible with fragile components.
FIG. 2 shows an alternative workholding device which provides support to the workpiece 5 during the de-bonding process. The gripping pin 3 is provided with an external supporting member 17. The gripping pin 3 is threadably coupled with the supporting member 17 and thus translates relative to the supporting member 17 as it is rotated by the motor 15 to de-bond the workpiece 5. The supporting member 17 has an annular shoulder 19 that prevents translation of the workpiece 5 during rotation of the gripping pin 3. However, again, it is necessary to prevent rotation of the workpiece 5. Furthermore, the translation of the gripping pin 3 may cause damage to the workpiece 5 even when supported by the annular shoulder 19.
As described above, mechanically breaking the bond can induce large stresses in the workpiece and is therefore unsuitable for fragile items. Furthermore, depending on the geometry of the workpiece, it may be difficult to direct laser radiation towards the adhesive.
It is also difficult to apply the adhesive to the gripping pin surface, particularly with the workpiece in situ. Consequently, the application of the adhesive may be inaccurate and messy.
Moreover, it is often difficult to determine whether the adhesive at the gripping pin surface has been adequately cured. Consequently, the duration of the curing process is typically set to be much longer than required to cure the adhesive so as to ensure complete curing. This increases the time taken to perform the manufacturing process, and thus the cost.
The process of removing residual adhesive from the gripping pins is time consuming and delays further operations on other workpieces. Furthermore, the removal of the residual adhesive may gradually damage the surface of the gripping pin, requiring the pins to be replaced frequently.
Furthermore, the LAAG system suffers from additional limitations, for example, in that                it is not possible to determine whether the workpiece is securely held before commencing the machining operation;        it can only hold flat workpieces;        the impact strength of the bond between the workpiece and the workholding may not be sufficiently high;        it requires expensive optical materials; and        it requires costly auxiliary equipment, such as a light guide and UV lamp.        
It is therefore desirable to provide adhesive fastening elements and methods of de-bonding which alleviate some or all of these problems and allows the workpiece to be de-bonded without causing damage to the workpiece.